Next installment of 9i new features, like always send hate mail to
/dev/null. Also if I've lied in something feel free to correct/flame me
for it.
The Saga continues:
Oracle 9i New Features: 9i Data Guard
So you have Oracle 8i standby database, what is new with 9i standby also known as Data Guard. Well in theory it can be totally automated. In 8i you had to put the
database in managed recovery mode(or you manually did recovery) on the standby
db. I hope no one was doing manual recovery every time a archive log was
shipped across. :)
Now with 9i dataguard, the standby database can be built either:
1. manually just like you did in 8i or
2. automagically with OEM Dataguard Wizard(crazy wizards for everything anymore). I personally couldnt get the wizard to work with 9.0.1.0. Does it
work now?, I'm not sure as I've not tested it as of recent. The manual
method works just fine.
Ok so you need to create a standby database, we're not going to cover that
here, why?, Mostly cause we're only going to cover the new aspects, you all can
read the docs as well as I can to build the db.
On to the new stuff: Your standby database in 8i was only in what we call "delayed" protection
mode, which means there is a delay from the time the logs are completed on the
primary database before they are appliedto the standby database. Now with
9i, you have 3 other modes:
Guaranteed protection: indicates that primary database modifications are
available to the standby database, up to the last committed transaction.
The standby database cannot diverge from the primary database at all, and no
data can be lost. If a standby database is unavailable, processing automatically
halts on the primary database as well.
Instant Protection: With instant protection, the standby database may
temporarily diverge from the primary database, but upon failover to the standby
database, the databases can be synchronized, and no data will be lost.
Rapid Protection: With rapid protection, the log writer process transmits
redo logs to the standby sites. Use this mode when availability and performance
on the primary database are more important than the risk of losing a small
amount of data.
An important note is that in Guaranteed and Instant protection mode that
the logs are written in SYNCHRONOUS mode to the standby site. Rapid and Delayed
mode are ASYNC writes.
So this begs to ask what is the difference between Guaranteed and
Instant? Well in Guaranteed mode the logs are applied and there is no data
divergence from the primary and standby db. Whereas Instant mode there can
be data divergence but upon failover there will be
NO data loss. So since we wont have data loss, there is a new way to "failover" to a
standby database, called switchover.
You can literally be able to switch to a standby database and switchback to
the original primary WITHOUT having to reinstantiate(ie: rebuild) the
primary.
A very nice feature, this gives you the capability to do rolling upgrade of
OS <-----NOTICE, NOT ORACLE BINARIES YET, ONLY THE OS.
In 8i you could only do a failover(which required rebuilding the primary),
you can still do a failover in 9i if you've lost the primary db in some disaster
like the computer room caught on fire.
Now for the automated part:
There is the dataguard manager/broker, it can be run from command
line($ORACLE_HOME/bin/dgmgrl) as well as from OEM. This gives you the
luxury of oracle doing all the work to maintain consistency between the primary
and standby database. Way too much to cover here but it handles
automatically applying logs, notification of down standby, etc.
Feel free to ask anything about Dataguard to me directly at
[EMAIL PROTECTED] Joe
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- RE: {9i New Features: DataGuard} JOE TESTA
- RE: {9i New Features: DataGuard} Arun Chakrapani
- RE: {9i New Features: DataGuard} Arun Chakrapani